1. Field of the Invention
The present invention pertains to sleep disorder assessment and reporting, and, in particular, to a method and apparatus for generating and providing a visual representation of sleep quality based on electrocardiogram (ECG) signals.
2. Description of the Related Art
Sleep-Disordered breathing (SDB) describes a group of disorders characterized by abnormal respiratory patterns or the quantity of ventilation during sleep. It is a highly prevalent disease that remains under diagnosed.
Obstructive sleep apnea (OSA), the most common such disorder, is characterized by the repetitive complete or partial collapse of the pharyngeal airway during sleep and the need to arouse to resume ventilation. OSA affects at least 2% to 4% of the adult population and is increasingly recognized by the public. The high prevalence and wide spectrum of severity of OSA in adults have been well documented by several population-based cohort studies conducted in the United States, Europe, Australia, and Asia. Available data indicates that OSA prevalence is 2 to 3 times higher in patients with cardiovascular disease (CVD) than in reference normal populations. In addition, population-based epidemiological studies and observations of OSA patients have consistently shown a link between OSA and hypertension, heart failure, atrial fibrillation, myocardial infarction, nocturnal sudden death, and stroke.
Another kind of SDB, called central sleep apnea (CSA), is a neurological condition which causes the loss of all respiratory effort during sleep and is also usually marked by decreases in blood oxygen saturation. CSA has been linked to increases in heart failure, left ventricular dysfunction, and stroke. Mixed sleep apnea combines components of both CSA and OSA, where an initial failure in breathing efforts allows the upper airway to collapse.
Polysomnography is a definitive diagnostic technique that may be used in patients with suspected apnea. It often requires spending a night in a sleep laboratory during which multiple physiological variables are continuously recorded. The variables generally include sleep staging to collect a number of signals using several devices including an electroencephalogram (EEG), an electromyogram (EMG), an electrooculogram (EOG), a respiration (flow, effort, oxygen saturation) detection device, and a snoring detection device. With these signals, disordered breathing, in addition to its effect on sleep and oxygenation, can be precisely quantified.
Polysomnography is expensive because it requires overnight evaluation in sleep laboratories with dedicated systems and attending personnel. The cost and relative scarcity of diagnostic sleep laboratories contribute to the fact that sleep apnea is widely under diagnosed (it is estimated that more than 85% of patients with clinically significant and treatable OSA have never been diagnosed). Hence, techniques to screen patients for SDB with fewer and simpler measurements and without the need for a specialized sleep laboratory may be of benefit.
Several different such techniques have been proposed. Examples include the Epworth Sleepiness Scale, the Berlin questionnaire, overnight oximetry, and devices combining limited respiratory assessment, ECG, and oximetry. Specialized analysis of 24-hour ECG recordings also has been proposed as a possible screening tool. The most often currently used in clinical practice is overnight oximetry. None of these techniques, however, has proven to be a viable yet simple and cost effective solution for SDB screening.